The present invention relates to a pressure differential operated brake booster for use in a vehicle hydraulic braking system.
A conventional pressure differential operated brake booster typically includes a housing divided into a constant pressure chamber and a variable pressure chamber by a diaphragm assembly. A valve body is carried by the diaphragm assembly and extends through the rear wall of the housing. A valve mechanism is disposed within the valve body and includes a plunger operatively connected to an input rod. An output rod is operatively connected to the front end of the valve body through a reaction disk. The valve mechanism is operable to develop a pressure differential across the diaphragm assembly to thereby move the diaphragm assembly forward or in a direction toward the output rod. This movement of the diaphragm assembly is transmitted to the output rod while the resulting reaction force is fed back to the input rod through the reaction disk and the plunger.
With such an arrangement, the boost ratio is the ratio of the cross-sectional area of the abutting part of the plunger to the cross-sectional area of the abutting part of the output rod. Typically, the ratio of the operative surface areas is increased to provide a high boost ratio so that the boost increases relatively rapidly with increased driver input during an emergency, as shown in FIG. 7. In FIG. 7, the numeral J represents a so-called "jump-in" output. This output is obtained under the influence of a clearance formed between the reaction disk and the plunger. The larger the clearance, the higher the level of deceleration.
However, with such a high boost ratio, an undesirably high degree of output force is produced even when a brake foot pedal is lightly depressed during normal braking. This causes a deterioration in the operability of the brake.
To this end, there has been proposed reaction means composed of inner and outer disks as disclosed in Japanese laid-open patent publication No. 59-195460. When a light pedal force is applied, the inner disk is first brought into contact with an output rod so as to provide a low boost ratio. Where the inner disk has the same diameter as a plunger (see FIG. 3 of the '406 publication), a reaction force is wholly fed back to an input rod. Also, where a part of the inner disk abuts a valve body (see FIGS. 5 and 7 of the '460 publication), a substantial part of the reaction force is transmitted to the input rod. In either case, a driver suffers from uncomfortable feel during operation of the brake pedal.
Accordingly, it is an object of the present invention to provide a pressure differential operated brake booster which enables various changes in boost ratio without any deterioration in brake pedal feel.